1. Field of the Invention
The present invention relates to the field of power electronics. It refers to a method for feeding reactive power into an AC grid system, in the case of which method at least one line of the grid system has a compensation voltage produced for it, which compensation voltage is phase-shifted with respect to the current in the line and is injected into the line. The invention furthermore relates to an inverter for use in such a method.
2. Discussion of Background
In electrical power transmission systems, the distribution of the power flows is determined, without any control measures, by the impedance relationships of the transmission lines which are interconnected in a grid. The electrically effective impedance of a transmission line can be varied by using series compensators to inject reactive power, via series voltage injection. This measurement changes the complex voltage difference between the connection points, and thus the power flow via the line. Series compensation can be used to deliberately increase or reduce the load on lines, and to utilize the overall transmission capacity more effectively. This is not limited to three-phase transmission systems. Use in single-phase AC systems is also feasible.
In existing system designs, it is in general possible to distinguish between thyristor-switched or thyristor-controlled series compensation and static synchronous compensation. Thyristor-controlled series compensation comprises a plurality of series-connected modules, which are installed in each phase of a three-phase transmission line. Owing to the thyristor technology that can be used, a parallel circuit comprising a capacitance and a controlled (thyristor-switched) inductance is required per module in this case. A component, which may be designed by way of example as a varistor, is also required to protect this parallel circuit against overloading. The disadvantage of this known system configuration is essentially that the semiconductor switches are mains-commutated in operation, which results in a very limited operating range.
Static synchronous compensators work electrically by injecting an additional voltage (compensation voltage) at right angles to the line current (phase shift of 90.degree.). The injection of this additional voltage necessitates a transformer, which is connected in series with a transmission line that is to be compensated. The transformer that is required for injecting the additional voltage forms a comparatively high cost element in the overall costs of such a system.